在LINQ中分组

假设我们有这样一个类:

class Person { 
    internal int PersonID; 
    internal string car; 
}

我有一个这个类的列表:list <Person> persons;

这个列表可以有多个具有相同personid的实例，例如:

persons[0] = new Person { PersonID = 1, car = "Ferrari" }; 
persons[1] = new Person { PersonID = 1, car = "BMW"     }; 
persons[2] = new Person { PersonID = 2, car = "Audi"    };

有没有办法我可以通过PersonID分组，并获得他所有的汽车列表?

例如，预期的结果将是

class Result { 
   int PersonID;
   List<string> cars; 
}

分组之后，我得到:

results[0].PersonID = 1; 
List<string> cars = results[0].cars; 

result[1].PersonID = 2; 
List<string> cars = result[1].cars;

从我目前所做的来看:

var results = from p in persons
              group p by p.PersonID into g
              select new { PersonID = g.Key, // this is where I am not sure what to do

有人能告诉我正确的方向吗?

当前回答

试试这个:

var results= persons.GroupBy(n => n.PersonId)
            .Select(g => new {
                           PersonId=g.Key,
                           Cars=g.Select(p=>p.car).ToList())}).ToList();

但在性能方面，下面的做法在内存使用方面更好，更优化(当我们的数组包含更多的项，如数百万):

var carDic=new Dictionary<int,List<string>>();
for(int i=0;i<persons.length;i++)
{
   var person=persons[i];
   if(carDic.ContainsKey(person.PersonId))
   {
        carDic[person.PersonId].Add(person.car);
   }
   else
   {
        carDic[person.PersonId]=new List<string>(){person.car};
   }
}
//returns the list of cars for PersonId 1
var carList=carDic[1];

2016-05-26 05:51:49

其他回答

首先，设置关键字段。然后写上你的其他领域:

var results = 
    persons
    .GroupBy(n => n.PersonId)
    .Select(r => new Result {PersonID = r.Key, Cars = r.ToList() })
    .ToList()

2019-05-06 19:06:21

另一种方法可以选择不同的PersonId和组与人连接:

var result = 
    from id in persons.Select(x => x.PersonId).Distinct()
    join p2 in persons on id equals p2.PersonId into gr // apply group join here
    select new 
    {
        PersonId = id,
        Cars = gr.Select(x => x.Car).ToList(),
    };

或者与流畅的API语法相同:

var result = persons.Select(x => x.PersonId).Distinct()
    .GroupJoin(persons, id => id, p => p.PersonId, (id, gr) => new
    {
        PersonId = id,
        Cars = gr.Select(x => x.Car).ToList(),
    });

GroupJoin在第一个列表(在本例中是PersonId列表)中生成一个条目列表，每个条目在第二个列表(人员列表)中生成一组已连接的条目。

2019-06-05 08:23:33

我想给出一个在c#中使用GroupBy的复杂查询的例子

    var records= (from x in db.Table
                        select new
                        {
                            x.Id,
                            x.ProgramId,
                            ProgramName = x.Program.Name,
                            Version= x.Program.Version,
                            x.Date,
                            x.Maks
                        });
    // I take two date(openDate and closeDate)
    var try= records
        .Where(x => x.Date>= openDate && x.Date<= closeDate)
        .GroupBy(x => new
        {
            x.ProgramId,
            MonthYear=x.Date.Value.Month + "-" + x.Date.Value.Year,
                
        })
        .Select(x => new
        {                  
           ProgramName = x.Select(y => y.ProgramName).FirstOrDefault(),
           Version= x.Select(y => y.Version).FirstOrDefault(),
           x.Key.MonthYear,
           x.Maks.Max()
          })
        .ToList();
    datagridview1.DataSource = try;

2022-12-01 14:19:59

try

persons.GroupBy(x => x.PersonId).Select(x => x)

检查是否有人在你的列表中重复出现

persons.GroupBy(x => x.PersonId).Where(x => x.Count() > 1).Any(x => x)

2016-08-15 22:24:45

当然，你基本上想要:

var results = from p in persons
              group p.car by p.PersonId into g
              select new { PersonId = g.Key, Cars = g.ToList() };

或作为非查询表达式:

var results = persons.GroupBy(
    p => p.PersonId, 
    p => p.car,
    (key, g) => new { PersonId = key, Cars = g.ToList() });

基本上，组的内容(当被视为IEnumerable<T>时)是给定键的投影(在本例中为p.car)中任何值的序列。

有关GroupBy如何工作的更多信息，请参阅我在Edulinq上关于该主题的帖子。

(我在上面将PersonID重命名为PersonID，以遵循. net命名约定，在“大写复合词和常用术语”一节中特别指出了这一点。)

或者，你也可以使用Lookup:

var carsByPersonId = persons.ToLookup(p => p.PersonId, p => p.car);

然后你可以很容易地得到每个人的车:

// This will be an empty sequence for any personId not in the lookup
var carsForPerson = carsByPersonId[personId];

2011-09-06 19:46:13

在LINQ中分组

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